The invention disclosed herein pertains to a machine for forming stacks of articles In particular, the invention resides in an improved mechanism for stripping a stack of articles from a conveyor in a stacking machine without stopping the machine to facilitate the stripping operation. The individual stacks of articles are usually forwarded to a packaging machine which is set up to insert a predetermined number of articles comprising a stack into a package The new stripping device is especially efficacious for stripping stacks of pliant or supple articles such as sanitary pads and disposable diapers from the conveyor in the machine.
For the sake of brevity, the stacking machine will be called a stacker herein. Stackers for forming stacks of pliant articles have been in commercial use for years Generally, a stacker is positioned at the output of a machine which makes articles such as diapers or pads. The articles are conveyed into the stacker and when a predetermined number of the articles reaches a pad removal station, a quick acting mechanism ejects the stack without requiring any stopping or even slowing down of the conveyor. Hence, in most cases, conventional stackers have been able to take the output of articles from the production machine at whatever rate the machine produces the articles. However, the state-of-the-art machines which produce diapers, pads and other articles have much higher product output rates than predecessor machines. For instance, the last generation of pad making machines typically would be capable of producing as many as 650 pads per minute. The stacker technology was available for designing stackers which could form and remove stacks at a rate which matched the output from the pad forming machine. The latest pad forming machine designs are capable of producing pads at the rate of 800 per minute but the speed of stackers has not kept up so the production rate capabilities of the pad making machines have not been fully exploited.
One type of stacker is disclosed in U.S. Pat. No. 4,399,905. Basically, in this stacker three pairs of axially spaced apart sprockets have their axes of rotation arranged at the corners of a triangle. Pairs of conveyor chains run over the triangularly arranged sprockets and there are paddles on the conveyor chains which extend outwardly therefrom. The chain runs continuously. Pads are deposited on the paddles where the paddles are spread apart as a result of the chain going around one of the corners of the triangle. The pads, such as diapers, are carried on the paddles in series At a stack ejection station where the paddle carrying pair of chains becomes vertical, the paddles become horizontal so that there is one pad above another on the paddles. At this time, a stripper bar is projected transversely to the stack of superimposed pads for stripping the stack from the paddles. Two of the pairs of sprockets are movable vertically. One of the pairs is driven upwardly by a rotating cam and follower arrangement during the stripping operation such that the vertical run of the chain is lifted at the same rate at which it is translating downwardly so that motion of the pads in the stack which is about to be stripped is stopped relative to a fixed point off of the machine. As soon as the stack is stripped, the two pairs of sprockets are allowed to descend again to their lowermost position while the conveyor chain advances with the paddles on it to form a new stack at the stack discharge station. Forcible lowering of at least one pair of the sprockets is accomplished with an actuator in the form of a pneumatic work cylinder which must restore the one pair of sprockets at a rapid rate instantly after the stack is stripped. Experience has shown, however, that pneumatic cylinders or actuators become unstable and operate erratically at very high speeds. Consequently, the pneumatic cylinder is the principal cause for the stacking rate being limited in prior stackers.
Another type of stacker such as is illustrated in U.S. Pat. No. 4,927,322 also uses triangularly arranged pairs of sprockets on which a closed loop chain which carries paddles runs. In this design, a stripper bar is driven transversely of an accumulated stack of pads by means of a pair of pneumatic actuators or work cylinders. By appropriate timing, one of the work cylinders drives the vertical stripper bar transversely to the stack of pads on the superimposed paddles without an attempt to stop relative motion of the paddles. When the stripper bar is projected outwardly to strip the stack under the influence of one of the pneumatic actuators, the two actuators cooperate to cycle the stripper bar by moving it downwardly and inwardly and upwardly so it arrives behind the accumulating stack before the number of articles which compose a stack has accumulated at the stripping station. In this design the erratic and unstable operation of the pneumatic operators also limit the frequency at which stacks can be stripped from the stacker.